Voltage-controlled oscillators (VCOs) and other phase/frequency tunable oscillator are typically embedded into a feedback system, such as Phase/Frequency-Locked Loops (PLLs/FLLs), to generate a sinusoidal carrier at a desired frequency with a very high spectral purity (i.e., low phase noise). As an example, the sinusoidal carrier may be used as the local oscillator (LO) in transceiver architectures. In the specific case of a PLL, a very clean clock reference (normally generated from a crystal oscillator) is used to “lock” in phase the VCO to run at a frequency which may be an integer or fractional multiple of the crystal oscillator frequency (see FIG. 1). The VCO itself is typically much noisier relative to a crystal oscillator.
There are several parameters that may affect the performance of a free-running VCO in terms of phase noise. In particular, non-idealities (e.g., mismatches) may increase the noise up-conversion around the carrier. Several techniques have been proposed to improve the VCO noise performance. However, these techniques are often not practical, since they are effective only for a narrow range of oscillation frequencies or process/voltage/temperature (PVT) variations.
A feedback system 100, such as a PLL or an FLL, is depicted in FIG. 1, according to the prior art. The feedback system 100 includes a VCO 12 and a feedback loop 18, including a crystal oscillator 16, a phase/frequency detector and charge pump 20, a loop filter 24, and a frequency divider 22. Typically, the feed back loop 18 of a PLL-VCO uses the frequency divider 22 to provide a divided-down (by N) signal version fN of the output signal fout from the VCO 12. The circuit 100 compares the signal fN to the reference signal fref waveform from the local oscillator 16 (a crystal oscillator in the example). The circuit 100 then generates a control voltage Vc that tracks and corrects the frequency fluctuation of the VCO 12 due to noise. This noise reduction/cancellation occurs only for frequency offsets within the bandwidth of the output of the loop filter 24. Additionally, in the illustrated prior art circuit, the VCO noise is canceled after being generated by injecting equal but opposite frequency fluctuations.
Therefore, it would be useful to have a robust technique to minimize the VCO phase noise over wide oscillation frequency range and PVT variations.